1. Field of the Invention
The present disclosure relates to power transmission and more specifically to linear actuators for providing multiple, discrete, forces and recovering energy from loads handled by such actuators.
2. Description of the Related Art
A hydraulic actuator is a device which converts hydraulic energy into mechanical force or motion. Actuators may be defined as those with linear movement and those with rotary movement. Linear actuators may be further sub-divided into those where hydraulic pressure is applied to one side of a piston only (single acting) and capable of controlled movement in only one direction, and those where hydraulic pressure may be applied to both sides of the piston (double acting) and capable of controlled movement in both directions. Linear actuators may also be classified as single-ended, which have an extension rod on one end of the piston only, or double-ended, which have rods on both ends of the piston. Single-ended actuators are useful in space constrained applications, but unequal areas on each side of the piston results in asymmetrical flow gain which can complicate the control system. Double-ended actuators have the advantage of producing equal force and speed in both directions, and for this reason are sometimes called symmetric or synchronizing cylinders.
Hydraulic actuator cylinders receive their power from pressurized hydraulic fluid, which is typically oil that is pressurized by a hydraulic pump. In some applications, the cylinders are powered pneumatically by a gas such as air that is pressurized by a compressor. The hydraulic cylinder includes a cylinder barrel, inside of which a piston moves back and forth. The barrel is closed on one end by the cylinder bottom (also called the cap) and the other end by the cylinder head (also called the gland) where a connected piston rod comes out of the cylinder to engage a load. The piston has sliding rings and seals to contain the pressurized fluid and prevent leakage. The piston divides the interior volume of the cylinder into two chambers, the bottom chamber (cap end) and the piston rod side chamber (rod end/head end). Single-acting hydraulic cylinders produce forces in only one direction (in or out) and double-acting hydraulic cylinders produce forces in two directions (in and out).
Hydraulic actuators are sized for the largest load they are expected to encounter in service. Conventional hydraulic actuation systems are very often inefficient because the load and the actuator force are mismatched and a control valve must be used to throttle the high pressure working fluid flow to the actuator. This throttling action wastes pumping energy, produces heat, and reduces the overall efficiency of the system. These systems also have no way of capturing energy from a load force that is in the same direction as the motion of the piston, such as when a load is under the force of gravity.
What are needed are hydraulic actuation systems having variable displacements and energy recovery capabilities.